Pencils are normally formed by enclosing the marking core in wooden sheaths. The wood used must meet certain physical standards of strength, dimensional stability and sharpenability. The present commercial process for making pencils is a multistep one, time consuming and relatively expensive. Substitutes for the wooden sheaths as well as for the involved manufacturing process have long been sought. It is, however, difficult to form a sheath material which is satisfactorily bonded to the marking core, and which the user will accept as a substitute for the well-known wood sheath. This is due to the fact that the sheath must meet certain desired criteria including sharpenability, adequate stiffness, good flexural or breaking strength, relatively low density and bondability to the marking core.
Many attempts have been made to manufacture pencil sheaths from a material other than wood. It has been proposed to manufacture pencil sheaths by extrusion and subsequent drying of an aqueous pulp of wood or paper with a suitable binder, or to tightly wrap the marking core with paper and the like. In the first instance, such attempts have been unsuccessful because the necessity of expelling large amounts of water from the extrudate gives rise to warped pencils and loose cores. Where pencils have been made by wrapping the core, problems have arisen in adhering the wrapping material to the core, as well as adhering the layers of wrapping material to each other, and in producing a straight, properly aligned sheath that couled be readily sharpened by a penknife or mechanical sharpening device.
U.S. Pat. Nos. 2,988,784 and 2,790,202 disclose a sheath composition, a method, and an apparatus for extruding a sheath around a marking core to make a so-called scissible writing instrument. The sheath composition disclosed is a mixture of a main ingredient (wood flour and the like), a waxy substance such as chlorinated naphthalene, a thermoplastic binder such as polyvinyl chloride, a plasticizer for the binder, and a lubricant. The sheath composition is blended and used in a completely dry condition, this being the patentee's way of avoiding the difficulties encountered in the prior art compositions and methods.
In the method and apparatus of U.S. Pat. No. 2,790,202, techniques and means are used to extrude the sheath around leads fed along a vertically-oriented axis. Cooling of the sheath is accomplished before the sheathed lead leaves the die nozzle.
The composition, method and apparatus of U.S. Pat. Nos. 2,988,784 and 2,790,202 present certain inherent difficulties and limitations. Among such difficulties are the need for working with completely dry sheath material, the need for using an adhesive to bond the core to the sheath, and the necessity to accurately control the radial component of pressure in relation to the axial component of pressure on the sheath material to overcome the back pressure in the die brought about through the vertical feeding of abutting core lengths. To the best of applicants' knowledge, this prior art teaching which overcomes some of the disadvantages and limitations of the art preceding it has a number of limitations of its own, which have apparently prevented it from achieving any commercial success.